This invention relates to heterologous G protein-coupled receptor expression constructs, yeast cells expressing such receptors, vectors useful for making such cells, and methods of making and using same.
The actions of many extracellular signals, for example: neurotransmitters, hormones, odorants and light, are mediated by receptors with seven transmembrane domains (G protein-coupled receptors) and heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins). G proteins are comprised of three subunits: a guanyl-nucleotide binding xcex1 subunit; a xcex2 subunit; and a "Ugr" subunit [for review, see Conklin, B. R and Bourne, H. R. (1993 Cell 73, 631-641]. G proteins cycle between two forms, depending on whether GDP or GTP is bound to the a subunit. When GDP is bound, the G protein exists as a heterotrimer, the Gxcex1xcex2"Ugr"complex. When GTP is bound, the a subunit disassociates, leaving a Gxcex2"Ugr" complex. Importantly, when a Gxcex1xcex2"Ugr" complex operatively associates with an activated G protein coupled receptor in a cell membrane, the rate of exchange of GTP for bound GDP is increased and, hence, the rate of disassociation of the bound Gxcex1 subunit from the Gxcex2"Ugr" complex increases. The free Gxcex1 subunit and Gxcex2"Ugr" complex are capable of transmitting a signal to downstream elements of a variety of signal transduction pathways. This fundamental scheme of events forms the basis for a multiplicity of different cell signaling phenomena. For a review, see H. G. Dohlman, J. Thorner, M. Caron, and R. J. Lefkowitz, Ann. Rev. Biochem, 60, 653-688 (1991). G protein-mediated signaling systems are present in organisms as divergent as yeast and man. The yeast Saccharomyces cerevisiae is utilized as a model eukaryotic organism. Due to the ease with which one can manipulate the genetic constitution of the yeast Saccharomyces cerevisiae, researchers have developed a detailed understanding of many complex biological pathways. It has been demonstrated in numerous systems that the evolutionary conservation of protein structure is such that many heterologous proteins can substitute for their yeast equivalents. For example, mammalian Gxcex1 proteins can form heterotrimeric complexes with yeast Gxcex2"Ugr" proteins [Kang, Y. S., Kane, J., Kurjan, J., Stadel, J. M., and Tipper, D. J. (1990) Mol. Cell. Biol. 10, 2582-2590). The G protein coupled receptors represent important targets for new therapeutic drugs. Discovery of such drugs will necessarily require screening assays of high specificity and throughput. For example, therapeutic intervention in the somatostatin growth hormone axis requires new chemical agents that act in a somatostatin receptor subtype-selective manner. The somatostatin receptor (SSTR) is a prototype of the seven transmembrane-domain class of receptors in mammalian cells. The cyclic tetradecapeptide somatostatin, first isolated from hypothalamus and shown to be a potent inhibitor of growth hormone release from the anterior pituitary, has been shown to have broad modulatory effects in CNS and peripheral tissues. In response to binding of somatostatin, SSTR activates a heterotrimeric G protein, which in turn modifies the activity of a variety of effector proteins including but not limited to adenylate cyclases, ion channels, and phospholipases. The effects of somatostatin are transduced through the action of gene products encoded in five distinct receptor subtypes that have recently been cloned [Strnad, J., Eppler, C. M., Corbett, M., and Hadcock, J. R. (1993) BBRC 191, 968-976; Yamada, Y., Post, S. R., Wang, K., Tager, H. S., Bell, G. I., and Seino, S. (1992) Proc. Natl. Acad. Sci. USA 89, 251-255; Meyerhof, W., Paust, H. J., Schonrock, C., and Richter, D. (1991); Kluxen, F. W., Bruns, C., and Lubbert, H. (1992) Proc. Natl. Acad. Sci. USA 89, 4618-4622; Li, X. J., Forte, M., North, R. A., Rose, C. A., and Snyder, S. (1992) J. Biol. Chem. 267, 21307-21312; Bruno, J. F., Xu, Y., Song, J., and Berelowitz, M. (1992) Proc. Natl. Acad. Sci. USA 89, 11151-11154; O""Carrol, A. M., Lolait, S. J., Konig, M., and Mahan, L. (1992) Mol. Pharmocol. 42, 939-946]. Screening assays utilizing yeast strains genetically modified to accommodate functional expression of the G protein-coupled receptors offer significant advantages in research involving ligand binding to the somatostatin receptor, as well as a host of other receptors implicated in various disease states.
A first aspect of the present invention is directed to expression vectors and yeast cells transformed therewith, containing a first heterologous nucleotide sequence which encodes for a G protein-coupled receptor, for example, the somatostatin receptor, and a second nucleotide sequence which encodes for all or a portion of a G protein xcex1xcex2"Ugr" complex. In certain embodiments, all or a portion of a nucleotide sequence encoding for a heterologous G protein a subunit is fused to a nucleotide sequence from the yeast G protein a subunit. In certain preferred embodiments, the expression vectors and transformed cells contain a third heterologous nucleotide sequence comprising a pheromone-responsive promotor and an indicator gene positioned downstream from the pheromone-responsive promoter and operatively associated therewith. The vectors and cells may further contain several mutations. These include 1) a mutation of the yeast SCGl/GPAl gene, which inactivates the yeast Gxcex1 protein, facilitating interaction of the heterologous receptor with the G protein; 2) a mutation of a yeast gene to inactivate its function and enable the yeast cell to continue growing in spite of activation of the pheromone response signal transduction pathway, preferred embodiments being mutations of the FAR1 and/or FUS3 genes; and, 3) a mutation of a yeast gene, the effect of the which is to greatly increase the sensitivity of the response of the cell to receptor-dependent activation of the pheromone response signal transduction pathway, preferred genes in this regard being the SST2, STE50, SGV1, STE2, STE3, PIKI, AFRI, MSG5, and SIG1 genes.
A second aspect of the present invention is a chimeric expression construct and yeast cells transformed therewith comprising a first nucleotide sequence encoding for a yeast G protein coupled receptor in operative association with a heterologous nucleotide sequence which encodes for a heterologous G protein coupled receptor. The constructs and cells may contain a second heterologous nucleotide sequence comprising a pheromone-responsive promoter and an indicator gene positioned downstream from the pheromone-responsive promoter and operatively associated therewith. The constructs and cells may further contain several mutations. These include 1) a mutation of a yeast gene to inactivate its function and enable the yeast cell to continue growing in spite of activation of the pheromone response signal transduction pathway, preferred embodiments being mutations of the FAR1 and/or FUS3 genes; and, 2) a mutation of a yeast gene, the effect of the which is to greatly increase the sensitivity of the response of the cell to receptor-dependent activation of the pheromone response signal transduction pathway, preferred genes in this regard being the SST2, STE50, SGV1, STE2, STE3, PIKL, AFRI, MSG5, and SIGI genes. A productive signal is detected in a bioassay through coupling of the heterologous receptor to a yeast protein.
A third aspect of the present invention is a method of assaying compounds to determine effects of ligand binding to the heterologous receptors by measuring effects on cell growth. In certain preferred embodiments, yeast cells of the kind described above are cultured in appropriate growth medium to cause expression of heterologous proteins, embedded in agar growth medium, and exposed to compounds applied to the surface of the agar plates. Effects on the growth of embedded cells are expected around compounds that activate the heterologous receptor. Increased growth may be observed with compounds that act as agonists, while decreased growth may be observed with those that act as antagonists.